The Collaborative International Dictionary of English v.0.48:

Heathen \Hea"then\ (h[=e]"[th]'n), a.
   1. Gentile; pagan; as, a heathen author. "The heathen
      philosopher." "All in gold, like heathen gods." --Shak.
      [1913 Webster]

   2. Barbarous; unenlightened; heathenish.
      [1913 Webster]

   3. Irreligious; scoffing.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Heathen \Hea"then\ (h[=e]"[th]'n; 277), n.; pl. Heathens
   (-[th]'nz) or collectively Heathen. [OE. hethen, AS.
   h[=ae][eth]en, prop. an adj. fr. h[=ae][eth] heath, and
   orig., therefore, one who lives in the country or on the
   heaths and in the woods (cf. pagan, fr. pagus village); akin
   to OS. h[=e][eth]in, adj., D. heiden a heathen, G. heide,
   OHG. heidan, Icel. hei[eth]inn, adj., Sw. heden, Goth.
   hai[thorn]n[=o], n. fem. See Heath, and cf. Hoiden.]
   1. An individual of the pagan or unbelieving nations, or
      those which worship idols and do not acknowledge the true
      God; a pagan; an idolater.
      [1913 Webster]

   2. An irreligious person.
      [1913 Webster]

            If it is no more than a moral discourse, he may
            preach it and they may hear it, and yet both
            continue unconverted heathens.        --V. Knox.
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   The heathen, as the term is used in the Scriptures, all
      people except the Jews; now used of all people except
      Christians, Jews, and Mohammedans.
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            Ask of me, and I shall give thee the heathen for
            thine inheritance.                    --Ps. ii. 8.

   Syn: Pagan; gentile. See Pagan.
        [1913 Webster]

WordNet (r) 3.0 (2006):

heathen
    adj 1: not acknowledging the God of Christianity and Judaism and
           Islam [syn: heathen, heathenish, pagan, ethnic]
    n 1: a person who does not acknowledge your god [syn: heathen,
         pagan, gentile, infidel]

The Devil's Dictionary (1881-1906):

HEATHEN, n.  A benighted creature who has the folly to worship
something that he can see and feel.  According to Professor Howison,
of the California State University, Hebrews are heathens.

    "The Hebrews are heathens!" says Howison.  He's
        A Christian philosopher.  I'm
    A scurril agnostical chap, if you please,
        Addicted too much to the crime
        Of religious discussion in my rhyme.

    Though Hebrew and Howison cannot agree
        On a _modus vivendi_ -- not they! --
    Yet Heaven has had the designing of me,
        And I haven't been reared in a way
        To joy in the thick of the fray.

    For this of my creed is the soul and the gist,
        And the truth of it I aver:
    Who differs from me in his faith is an 'ist,
        And 'ite, an 'ie, or an 'er --
        And I'm down upon him or her!

    Let Howison urge with perfunctory chin
        Toleration -- that's all very well,
    But a roast is "nuts" to his nostril thin,
        And he's running -- I know by the smell --
        A secret and personal Hell!
                                                           Bissell Gip